This invention relates to a system, and a method, for using an optical backplane to provide ultra high frequency optical interconnections amongst microprocessors. The backplane is comprised of an optical waveguide network and magneto-optical routers.
Th speed of computing devices, such as electronic processors, has been steadily increasing. Processing speed is accompanied by a need for rapid communication amongst processing units. The communication bandwidth requirements of microprocessors (the words processing unit and microprocessor will be used interchangeably) are roughly proportional with speed, and chip cycle times are reaching into the GHz domain. The central computing complexes of large computers comprise of many individual microprocessors, packaged into multi chip modules (MCM), combining their individual performance in a fashion that is transparent to end user. This is only possible if the individual microprocessor chips communicate with each other at sufficiently high bandwidth. Another arena where processor unit to processor unit communication is of concern is the massively parallel computing approach. In such computers hundreds, or thousands, of individual processors, each possibly comprising of several microprocessors, have to be all interconnected at high speeds. A major packaging challenge of computing systems always has been the communication infrastructure, the so called wiring backplane of sufficient bandwidth. The way this is presently done is to have sufficient number of metal wiring in the backplane, connecting the processors to one another. However, as data rates increase metal interconnects between chips, or between multi chip modules, are reaching their limits. Communication between processor chips is starting to be a performance bottleneck. Metal interconnects suffer from loss, cross talk, excessive power requirements, all limiting the maximum achievable bandwidth. As a result of such difficulties optical interconnections are now being seriously considered to take the place of metal wiring.
Optical interconnects have the distinct advantage of almost limitless bandwidth, no cross talk, and low loss. However, the actualization of a purely optical backplane hitherto faced formidable obstacles. There are problems with the integration of lasers, detectors, and waveguides into necessarily small spaces afforded in microprocessor technology. There is also the problem of how to direct light pulses along an optical network at GHz speeds. Then, there is the problem of process integration, namely the difficulty of the processing technology needed to incorporate lasers, detectors, and waveguides into a CMOS technology framework.
The object of this invention is an optical backplane and methods of its use in an electronic processing system. Such a processing system comprises of a large number of microprocessors, with the backplane providing connections amongst the processing units. The processing system can be a single processor in a multi chip embodiment, in which case the processing units are individual chips, or the processing system itself can be a multi processor, in which case the individual processing units again can be chips, or can alternatively be MCMs. Or, one can have combinations of these, depending on the particulars of a system, as one skilled in the art would observe.
It is a further object of the invention to use thin film technology for creating the optical backplane, such a technology being similar and compatible to that used in CMOS technology, whereby such an optical backplane can be integrated with CMOS technology.
It is yet a further object of the invention to provide for routers in such an optical network. These routers are based on magneto-optical polarization rotator and polarization beam splitter combinations.
It is yet a further object of the invention to provide for the whole optical interconnection network, based on planar, ridge, or cylindrical waveguides. Such waveguide types are well known in the appropriate arts. Also, for providing apparatus and method for controlling the routers in such an optical network. The routers allow establishing of communication amongst processing units at a speed commensurate with the bandwidth requirements. Communication amongst processing units can mean interconnecting any two unit, or to allow for a broadcasting mode, where one processing unit simultaneously transfers data to more than one other unit, or possibly to all of the other units.
The light in the network originates when electrical signals from each microprocessor drive an array of lasers, preferably Vertical Cavity Surface Emitting Lasers (VCSEL""S). Laser light is polarized, and when such light is steered into the waveguide network through the optical devices that operationally connect the processing units to the network, it enters the waveguides in a polarized state. The operation of the optical routers is based on the fact the light in the network is polarized. There are several ways, based on polarization beam splitters, to direct light into differing optical paths depending on the polarization angle of the light. If such a polarization beam splitter is preceded by an optical element which is capable to controllably set the polarization angle, one has achieved an optical router which is part of an optical waveguide network. In the preferred embodiment such an optical element, which controllably sets the polarization angle, is a magneto optic rotator (MOR). In a MOR the waveguiding layer has magnetic properties, and depending on its magnetization state it rotates the polarization angle of the guided light. In a preferred embodiment such a magneto-optically active layer comprises of Yttrium Iron Garnet (YIG).
When the polarized light passes through a YIG waveguide segment, the polarization of the incident light can be converted from TE to TM mode (horizontal to vertical polarization), or vice versa, depending on the magnetization within the YIG. In a preferred embodiment this mode conversion is done in two steps, using two sections of YIG material. In the first step the incident polarization is rotated by +45xc2x0 or xe2x88x9245xc2x0. A second section of YIG waveguide has its magnetization permanently aligned parallel to the direction of light propagation and gives a constant +45xc2x0 of rotation to the incident light, which has already been rotated by +45xc2x0 or xe2x88x9245xc2x0. This then gives a final angle of rotation of 90xc2x0 or 0xc2x0, depending on the choice made in controlling the magnetic field in the first section. One skilled in the art will observe that the operation of the routing scheme is not in need of polarization rotation angles which are exactly of the desired values. There is some latitude of having the polarization rotations accomplished to within a few percent of the exact desired values.